This invention relates generally to power transmission line protection systems, and more specifically concerns a line differential protection system for power transmission lines.
Modern power systems typically require high speed fault clearing to preserve the transient (short term) stability of the system and to provide better power quality by reduction in reduced voltage (voltage sag) duration. The most widely used fault protection systems satisfying such requirements for transmission lines, i.e. those power lines with nominal voltages of 115 KV and greater, are directional protection systems using directional comparison techniques. While the directional comparison approach has some advantages, including low channel (communication) requirements between relays positioned at the local and remote ends of the power line, along with inherent redundancy, it does require voltage values obtained from the power signal on the power line. Such systems experience problems (often severe problems) because of voltage errors or missing voltages caused by small voltage factors, including blown fuses in the system, problems with windings in the system voltage transformer (VT) devices and transient responses in the system capacitive coupled voltage transformers.
One alternative to directional comparison systems using voltage values is a current differential system, which uses only the electrical current value information from the power line. Current differential systems, also known as line differential systems, do not require voltage measuring devices, as they do not use voltage values in their fault determinations. Line differential systems are less sensitive to power swings and sudden load changes in the system and are generally less sensitive to or even immune from certain conditions on the line, including zero sequence mutual coupling effects and/or current reversals, among others. However, along with the advantages are several significant disadvantages, including reliance on high communication channel performance, which is required between the local and remote protective relays on the line. In addition, conventional line differential systems using phase current quantities are limited in their ground fault resistance coverage and also are a compromise to an extent in security under current transformer (CT) saturation conditions.
The present invention is a new line differential protection system which, while still dependent upon a communication channel, includes significant improvements relative to other system considerations, including high fault resistance coverage, improved operating characteristics and sensitivity, while at the same time maintaining power system security.
The present invention is a system for current differential protection for a three terminal power line configuration, comprising: means for determining selected current values present at each terminal of the three terminal line; means for processing the three selected current values in three successive processing operations using what we referred to as local terminal and remote terminal current values, wherein the current values at each one of the three terminals are processed in turn as a local currents and the current values at the other two terminals are in turn combined and processed as remote terminal currents, wherein each processing operation produces an output signal which is a trip signal or not in accordance with pre-selected processing criteria; and means for evaluating the results of the processing means such that when there is agreement between the three processing operations, the resulting system output signal is the same as that for the three processing operations and when there is disagreement, the output signal produced when the terminal with the maximum current values in the local terminal current is taken as the resulting system output.